Mine would be a mixture of Dwarf Fortress, IVAN, Elona, Armok 1's magic, and Dungeon Crawl. So pretty much, it would be Dwarf Fortress with an awesome magic/artifact system, a whole mess of object/spell/monster interaction, and a good, sensible way of upgrading the minions through teaching them magic, teaching them skills, and giving them better items. Then, once you've made your awesome dungeon, an upload system so everybody else can run heroes through it, if they wanted. It would probably be a little light on the "survival" aspect, at least in mid-game out, but it should have a strong focus on attracting, teaching, and training good minions. There'd be a good number of options for mundane items you can make and trade with various nations and groups, but the real fun begins when you start attracting mages. In addition to the pure mage who just studies the meta-physics of magic, how it works, and what it can do, there'd more importantly be your mages you teach a trade skill. From there, they can start making powerful magic items. Sure, you can have them enchant pre-made items, but while they would make an "iron sword +1" that way, if they made the item from scratch using their skills, you could have them make a "iron sword that heals you +1" or "flaming iron sword +1", depending on the magic they know, how good they are at the magic, and most importantly, how good they are at the trade skill.
Oh, and of course, a whole slew of crazy, rare-as-fuck randarts and fixedarts based on insane magic items and effects, randomly generated and seemingly=randomly-generated high-level beasts that your mages can create, somewhat to extremly random spell effects, and of course, randomly-generated enemies with different asperation and goals to conquer, ally, or trade with. For example, you might have an umbrella that makes it rain when it's open; a giant naga with another head and set of arms on the tail end; a spell that will attempt to turn the target into gold, which could either make the person a gold statue, a pile of gold coins, an entire gold golem, turn the target's items into gold, turn the target's LIMBS into gold, or even do weird-ass stuff like give the target the Mida's Touch; and your foes would be a nation of elves that have taken refuge in a mountain forest and ride giant eagles while shooting magically enchanted arrows; a barbarian tribe of orcs ruled by a known vampire orc; and a powerful were-cheetah with a whole army of cheetahs under his iron fist (litterally, his right hand is made of iron somehow), all of which were randomly generated from various premade parts stuck together in strange and unpredictable ways.
The strangest thing, of course, is that this is what Dwarf Fortress is going to become, so you know what? I'm happy. If I wanted something else, it would be the same thing with only the magic and magic items system, but it would be a 3d RTS with really smart AI. I do enjoy sitting back and letting two computer players kill each other in an RTS. Failing that, it would obviously be online, and I would enjoy watching games, if possible. I wouldn't mind it being 2d, of course, but 3d would be better in pretty much every way. As long as it's not psuedo-3d, I'm ok. You know what I mean, like Starcraft and Warcraft, I can't stand the sprite-based 3d games, it just looks strange. Maybe if the sprites were really high quality, I could stand it, I dunno.